Air ionization electrodes are used in equipments such as photocopiers, electrical air cleaners, air ionizers and ultrafine particle sensors. They are frequently embodied as thin-wire electrodes or needle-tip electrodes and are connected to a high voltage (HV) supply, which is set at a voltage (Vcor) that is sufficiently high to ionize the air in the direct vicinity of the ionization electrode. In case a positive HV is used, the ionization electrode effectively emits airborne positive ions. Negative ions are emitted when a negative HV is used. The emitted ions can attach themselves to airborne particles, thereby charging the particles. In air cleaners, particle charging is useful to increase the particle capturing efficiency in charged media filters positioned downstream from the particle charging section. Concerning air ionizers, emitted ions (often present as a bipolar mixture) serve to prevent the build-up of static charges on surfaces through charge neutralization. In ultrafine particle (UFP) sensors, emitted ions serve to charge particles in the airflow passing through the sensor. The UFP sensor subsequently determines the airborne particle concentration by measuring the particle-bound charge (see e.g. J. Marra; Journal of Nanoparticle Research (2010), Vol. 12, pp. 21-37).
An important requirement for an ionization electrode is that the total emitted ionization current remains constant in time. This is usually fulfilled by introducing an electronic feedback mechanism, which ensures that the voltage applied to the ionization electrode is always such that a constant pre-set ionization current is emitted.
A further important requirement for an ionization electrode is that the spatial ion emission density around the electrode exhibits cylindrical symmetry (in which case the wire or needle being the axis of a cylinder) and remains substantially unchanged in the course of time. This is especially important for UFP sensors to ensure a uniform and predictable degree of particle charging at all locations within their particle charging section. However, due to the gradual (non-uniform) deposition of contaminants from air on the ionization electrode, this requirement is not always guaranteed. The deposits or contaminants on the electrode may consist of deposited particulate species, but also of NH4NOx and SiO2 residues. A build-up of NH4NOx results from the oxidation of N2 into NOx within the corona plasma region around the ionization electrode and the subsequent reaction of NOx with NH3 gas in the presence of moisture to form the solid NH4NOx (x=2 or 3) salt. SiO2 is formed as a leftover from the oxidation of silicone-containing gases in the corona plasma region. These deposits are insulating in nature and their formation and growth on an ionization electrode is experienced to gradually change the spatial characteristics of the emitted ion density around the electrode. In case needle-tip electrodes are used, the plasma region wherein air ionization occurs is mostly confined to the electrode tip. Contaminating deposits are therefore predominantly found at or in direct proximity to the electrode tip. In case thin-wire electrodes are used, air ionization and thus also the formation of contaminating deposits occurs across the entire length of the wire. In UFP sensors the presence of such deposits/contaminants affect the particle charging behavior in the course of time, thereby reducing the reliability of these devices. This is a problem since such sensors rely on the correct interpretation of measured current signals into key characteristics of the UFP pollution, notably the UFP number concentration N and the average particle size dp,av. Eventually, small amounts of the deposit may be released back into air as nanoparticles under the influence of the local corona current, thereby further affecting the reliability of the readings of UFP sensors. This problem is quite serious when UFP measurements are carried out in indoor environments, which are always to some extent polluted with silicone-containing gases.
To deal with this contamination problem, the ionization electrode(s) may be manually cleaned from time to time. Further, a few cleaning devices have been suggested, such as specific brushes disclosed in U.S. Pat. No. 5,768,087, but the scope of their applicability is severely limited. Moreover, cleaning may be costly and time consuming. The installation of an activated carbon filter upstream of the ionization electrode may adsorb silicone gases from sampled air but is not acceptable for UFP sensors because the presence of such a filter also affects the UFP concentration in the sampled air which one wants to measure. An activated carbon filter is not effective for avoiding the deposition of particulate contaminants or of NH4NOx onto the ionization electrode. Thus, there is a need in the art for improved or alternative cleaning devices for air-ionization electrodes such as needle-tip or thin-wire electrodes.